The origins of thixotropy of fresh cement pastes

In this paper, we show that the thixotropic behavior of standard fresh cement pastes may find two origins. We first clarify the ambiguity in literature concerning the critical strain of fresh cement pastes, thanks to a detailed analysis of their macroscopic behavior. We show that the largest critical strain can be associated to the network of colloidal interactions between cement particles whereas the smallest critical strain can be associated to the early hydrates, which form preferentially at the contact points between cement grains. From the study of the structuration kinetics, we show that there exists a short term thixotropy due to colloidal flocculation along with a long term thixotropy (of practical interest) due to the ongoing hydrates nucleation. From dimensional considerations, we moreover suggest why the apparent yield stress of the mixture left at rest can be approximated as a linear function of time.